1. Field of the Invention
The present invention relates to a transformer, and in particular to a transformer voltage supply circuit thereof, applied to tubes, balancing the currents thereof.
2. Description of the Related Art
In the rapid development of display technology, liquid crystal display (LCD) monitors have been very common. Over traditional cathode ray tube (CRT) monitors, LCD monitors have had advantages of smaller thickness, fewer occupying space, and more stable screen without flicker. An LCD monitor has a backlight module, comprising lighting tubes driven by high voltage. Generally, this kind of lighting tube is driven by an inverter including a driving circuit and a high voltage transformer. To decrease the volume of LCD monitors, the transformer inside the inverter is designed as thin and small as possible.
Currently, many kinds of displays, for example, LCD monitors, prefer highly efficient, light, and smaller lighting tubes as backlight. Cold cathode fluorescent lamps (CCFL) have been commonly used, and, as dimensions of the monitor increase, backlight modules use a plurality of lighting tubes, rather than a single lighting tube, to supply satisfactory brightness.
In a conventional transformer of an inverter, primary windings and secondary windings are wound around a hollow bobbin with an iron core disposed inside. FIG. 1a shows an embodiment of the conventional transformer applied in an inverter. FIG. 1b is a cross-section of a bobbin with windings in the conventional transformer in FIG. 1a. 
As shown in FIG. 1a, the conventional transformer 10 of the inverter includes a first E-shaped iron core 122 and a second E-shaped iron core 121. The first iron core 122 and the second iron core 121 function together to form a closed magnetic circuit. In addition, the conventional transformer includes a bobbin 13. The bobbin 13 has a primary winding window 131 and a secondary winding window 133. A plurality of metal pins 135 at two ends of the bobbin 13 connect and weld conduction cords of the windings to a circuit board. A separator 132 is disposed between the primary winding window 131 and the secondary winding window 133. In addition, the secondary winding window 133 is divided into several winding areas by separators 134.
As shown in FIG. 1b, in the structure of the bobbin, the primary winding window 131 is used for a primary winding 141, and the secondary winding window 133 for a secondary winding 142. The secondary winding 142 has a relatively small diameter and a relatively larger winding number. When wound in multiple layers, the voltage difference between conduction cords in adjacent layers can be high enough to cause arcing. To avoid this, the separators 134 usually separate the secondary winding window 133 into several winding areas.
However, because the primary windings and the secondary windings are wound around the same bobbin, the conventional transformer can experience some problems.
For example, when only a single transformer drives more than two lighting tubes, the load power of the conventional transformer increases such that the temperature of the primary windings increases, raising the temperature of the transformer to unacceptable levels. While this problem can be solved by increasing the diameter of the conduction cords of the primary winding, the volume of the transformer increases accordingly, such that this is not an ideal solution.
FIG. 2 shows a conventional voltage supply circuit for lighting tubes. The voltage supply circuit includes a driving circuit 21, a transformer 22, capacitors C1 and C2, a balance circuit 23, and lighting tubes 251 and 252. The transformer 22 includes a primary winding 221, a secondary winding 222, and an iron core 223. The driving circuit 21 supplies a low voltage signal to the primary winding 221 of the transformer 22, and the secondary winding 222 inductively generates a high voltage signal to drive lighting tubes 251 and 252. Due to impedance and stray capacitance of the conduction cord, current through the lighting tubes 251 and 252 is not the same. Thus, the lighting tubes 251 and 252 have different brightness, thus degrading the display quality. A balance circuit 23 is then necessary to normalize current through the lighting tubes 251 and 252.
FIG. 3 shows another conventional voltage supply circuit for lighting tubes. The two conventional voltage supply circuits for lighting tubes in FIG. 3 and 2 differ in the disposition of the balance circuit 33, which is connected between the ground and the lighting tubes 251 and 252.
In conventional voltage supply circuits for lighting tubes, because the transformer 22 includes only two windings for high voltage and low voltage respectively, methods of driving the lighting tubes include serial tubes, parallel tubes, and multiple transformers. Serial tubes balance the current, but the transformer is still vulnerable to high voltage. An additional balance circuit is necessary when connecting lighting tubes in parallel. Multiple transformers increase cost and space used. Thus, none of the three methods provides an ideal solution.